1. Field of the Invention
This invention generally relates to an apparatus for topographical analysis of anatomical surfaces and, more particularly to such an apparatus enabling the tridimensional shape of partially reflecting anatomical surfaces to be spatially located. The disclosed apparatus can be specifically but not exclusively employed for topographical analysis of the cornea of a human eye.
2. Description of the Prior Art
The measurement of the shape and curvature of the human eye (corneal topography, cheratometry) is a technique which is being strongly developed in coping with the requirements of the recent refractive surgery techniques, as well as in view of the selection or manufacture of custom-fit contact lenses and for diagnosis of certain pathologies (such as keratoconus, traumas or post-surgical complications etc.). The backgrounds of such techniques are rather old. In 1619 Father Christopher Schneider compared to this effect the image of a window reflected upon the eye to the same image reflected upon balls of different diameters.
After studies of Jesse Ramsden (1796) and Helmotz, in 1881 Javal and Schiotz disclosed a cheratometer (or ophtalmometer), use of which is presently still largely widespread, enabling the average corneal curvature to be measured based upon the reflected image of a suitable reference sight line projected upon the eye, according to various meridian axes.
In the meantime, in 1880, Antonio Placido firstly disclosed the use of a disk having concentric white rings upon a black background, presently still known as Placido Disk. The reflected images are observed through a pin-point hole placed at the center of the disk and the shapes of the circles and their mutual distances enable a qualitative, but significant for a skilled person, analysis of the corneal curvature to be made in all areas involved in visual activities.
An alternative approach, mainly employed in surgery operations, provides for a "disposable" cylinder to be placed upon the eye and having alternated black and white cylindrical reference sight lines or so-called "mires" internally formed therein with a height suitably increasing starting from the eye (as taught by Dr. William F. Maloney).
Subsequently, various illuminating systems (as in U.S. Pat. No. 3,248,162, No. 4,772,115 and No. 5,018,850) and a photographic camera to record the measurements (see in U.S. Pat. No. 3,598,478 and No. 3,797,921) have been added to the Placido Disk. The pictures so obtained can also be picked up by a TV camera and processed by a computer (LSU Corneal Topography System and Photocheratoscope Nidek PKS-1000).
A subsequent step provided for arranging the TV camera directly behind the reference sight lines. In a specific variant, the reference sight lines maintain a shape similar to the shape they have in a Placido Disk (retro-illuminated disk or dome). In other variants (such as in U.S. Pat. No. 3,598,478 and No. 4,863,260) the shape of the illuminating device is such that it can protrude into the superciliary arch.
An advantage of this second approach, of course, is that it provides a more compact apparatus which is adapted to project the most external reference sight lines from a significantly larger angle and with less shadows (eyebrow, nose, etc.). The weakness point of this approach, however, is that even the smallest error in the position of the eye when the measurement is carried out causes a noticeable error when the resulting data are analysed. For instance, when the eye is moved closer by 2 mm, an error is caused corresponding to a variation from 43 to 42 diopters, while in topography an accuracy of 1/4 diopter is required.
The technique as described in U.S. Pat. No. 4,863,260 (and in U.S. Pat. No. 5,018,850), for instance, uses two intersecting lasers and this makes the measurement operation extremely sensitive and dependent on the user's hand, without even the possibility to know the accuracy with which the measurement has been performed (in view of the fact that the lasers are turned off when the picture is taken).
The broad object of this invention is to provide an apparatus for topographical analysis, particularly for the cornea, wherein all drawbacks of the prior art are eliminated, particularly in so far as the accuracy and repeatability of the measurements, as well as the independence and the oversensitivity of the involved manual operations are concerned.
A particular object, therefore, is to provide an apparatus adapted to pick up and to process the image of an eye only when the eye certainly is in a reference position, thereby assuring a greater than usual accuracy.